1. Field of the Invention
The present invention relates to an organic light emitting diode display and a method of fabricating the same. More particularly, the present invention relates to an organic light emitting diode display and a method of fabricating the same, which may reduce or prevent deterioration in image quality and may simplify the manufacturing process.
2. Description of the Related Art
An organic light emitting diode display displays an image by driving N×M organic light emitting elements with a voltage or current. The light emitting elements are emissive display elements that electrically excite an organic compound to emit light.
Since the organic light emitting element has diode characteristics, the organic light emitting element is also referred to as an organic light emitting diode (OLED). The organic light emitting element may include an anode electrode that is a hole injection electrode, an organic layer including a light emitting layer, and a cathode electrode that is an electron injection electrode.
When holes and electrons are injected into the organic layer, exitons obtained by combining the injected holes and electrons may be transitioned from an excited state to a ground state, so that light is emitted from the organic layer.
The OLED display including the aforementioned organic light emitting elements may include a substrate having a pixel driving circuit. The substrate may include a buffer layer, and the pixel driving circuit including a plurality of thin film transistors (TFTs) may be formed on the buffer layer.
The pixel driving circuit may include at least two TFTs and a storage capacitor for three, e.g., red, green, and blue, sub-pixels constituting an organic light emitting element.
A first TFT of the two or more TFTs may serve as a switching TFT for selecting an element to emit light from among a plurality of organic light emitting elements. In general, a first gate electrode of the first TFT may be coupled to a scan line, a first source electrode thereof may be coupled to a data line perpendicular to the scan line, and a first drain electrode thereof may be coupled to a lower electrode of the storage capacitor.
A second TFT of the two or more TFTs may serve as a driving TFT for applying a driving voltage used to emit light from the organic layer of the selected organic light emitting element. In general, a second gate electrode of the second TFT may be coupled to the lower electrode of the storage capacitor, a second source electrode thereof may be coupled to a power supply line, and a second drain electrode may be coupled to the anode electrode.
A process of forming the aforementioned pixel driving circuit and the organic light emitting element on a sub-pixel region may be as follows.
In order to form the pixel driving circuit, the buffer layer may be formed at a side of the substrate, and first and second semiconductor layers may be formed on first and second TFT regions of the buffer layer, respectively. The first and second semiconductor layers may include source, drain, and channel regions, and may be made of, e.g., polysilicon.
In addition, a single first semiconductor layer and a single second semiconductor layer may be formed in the sub-pixel region. Therefore, twice as many semiconductor layers may be formed in the sub-pixel region than on a remainder of the substrate.
Then, a gate insulating layer may be formed, and gate wire lines may be formed on the gate insulating layer. The gate wire lines represent conductive materials formed on the gate insulating layer. Here, the gate wire lines represent the first gate electrode of the first TFT, the second gate electrode of the second TFT, the scan line connected to the first gate electrode, and the lower electrode of the storage capacitor connected to the second gate electrode.
Next, an interlayer insulating film having a plurality of via holes may be formed on the aforementioned structure, and source/drain wire lines may be formed on the interlayer insulating film. The source/drain wire lines represent conductive materials formed on the interlayer insulating film.
Here, the source/drain wire lines represent the first source and drain electrodes of the first TFT, the second source and drain electrodes of the second TFT, the data line coupled to the first source electrode, the power supply line coupled to the second source electrode, and an upper electrode of the storage capacitor coupled to the power supply line. In this case, the first drain electrode is also coupled to the lower electrode of the storage capacitor.
Next, a planarization layer may be formed on the aforementioned structure, and the organic light emitting element may be formed on the planarization layer. The organic light emitting element may include first and second pixel electrodes and an organic layer interposed between the electrodes.
Here, in general, the first pixel electrode disposed below the second pixel electrode may serve as an anode electrode, and the second pixel electrode may serve as a cathode electrode. The first pixel electrode may be coupled to the second drain electrode of the second TFT.
In addition, the organic layer may be constructed as a multilayer structure including an electron transport layer ETL and a hole transport layer HTL. The organic layer may further include an electron injection layer EIL and a hole injection layer HIL.
In the organic light emitting diode display having the aforementioned construction, the source/drain wire lines are generally composed Ti/Al/Ti. Al has a specific resistance of 2.2×10-6 Ω·cm. Accordingly, the power supply line including Al has a relatively high specific resistance.
Therefore, when IR drop occurs along the power supply line, problems, e.g., lack of brightness uniformity and cross-talk, due to the IR drop occur. As the size of the OLED display increases, these problems increase.
In addition, there is a problem in that the source/drain wire lines and the first pixel electrode are formed on different layers from each other.
Accordingly, a fine metal mask for forming the source/drain wire lines on the interlayer insulating film and a fine metal mask for forming the first pixel electrode on the planarization layer are separately provided. Therefore, layer-forming processes using the masks have to be separately performed. Therefore, cost reduction of the OLED display is limited.